SMC is used in the manufacture of converted articles not only in the automotive field (bumpers, tailgates, etc) but also in the marine (boat hulls) or electronics (casings) fields. SMC is generally composed of a crosslinkable polymeric resin, in particular an unsaturated polyester, reinforcing fillers such as glass fibres, and various other additives in minor quantities.
The SMC is ordinarily prepared by laying fibres on a layer of unsaturated polyester resin, which is itself supported on a moveable film, composed in general of polyethylene or of polyamide. Another film of the same kind is subsequently laid over the resin/reinforcing filler system in order to form a sandwich composite structure between two films. The sandwich subsequently passes through a series of kneading and compacting rollers and is generally wound up in the form of large rolls.
It is then stored before subsequent forming. During the storage period, the polyester resin undergoes partial crosslinking, which brings about an increase in the viscosity of the SMC, until it attains a consistency making it suitable for moulding. SMC users, generally moulders, cut an appropriately sized section from the roll, remove the support film by peeling and place the SMC in a heated mould for simultaneous forming and full cure. Thus composite compounds in the form of an SMC sandwich find ready application in compression moulding procedures.
The BMC (Bulk Moulding Compound) technology is similar except that the polyester to be crosslinked is either in a thick layer between two films or is in bulk in a drum packed with a film. The SMC and BMC compositions contain styrene.
Three properties relating to the sandwich film are of crucial importance for SMC manufacturers and users.
The first relates to the permeability of the peelable film to styrene. It is necessary for this peelable film to have a very low permeability to styrene in order to avoid the loss of monomeric styrene which plays the role of crosslinking agent in SMC. This loss of monomeric styrene is also damaging for the health of individuals and for the environment during operations for the manufacture of SMC and for its storage.
The second property relates to the ease of peeling of this film on the polyester structure, so that no residual film remains on the structure and to avoid the risks of tearing this film during operations of manufacture and forming of SMC.
Finally, the uptake of moisture and the water permeability of these peelable films should be very low so that the quality of the polyester resin, which is very sensitive to water, is not impaired during operations for the manufacture of SMC, the storage of the polyester or the forming of SMC.
The prior art described numerous mono- or multilayer films which have barrier properties and which can be used in the SMC technology, but also in other technologies such as fumigation. Patent EP 506515 describes films consisting of a polyamide and polyolefin blend for SMC. Patent EP 766913 describes the use of films consisting of a polyamide and polyolefin blend for fumigation. The fumigation of soils consists in treating the soils by injecting gas to a depth of about 0.5 or 1 m and then the soil to be treated is covered with a film so that the gas remains in the soil for longer, which makes it possible to reduce the quantities of gas to be used. Patent EP 990515 describes a film comprising a central polyolefin layer and two outer layers in the form of a polyamide/polyolefin alloy having a surface tension which is only slightly different from that of the polyolefin of the alloy. These films are useful in the SMC technology and in fumigation. Patent EP 997268 describes a mono- or multilayer film comprising metallocene polyethylene at least a first layer of a polyamide and polyethylene blend, optionally a second polyethylene layer in which the metallocene polyethylene is in the first and/or in the second layer. These films are useful in the SMC technology and in fumigation. None of these documents describes polyamide and polyolefin blends containing nanofillers or their barrier properties.
It has now been found that the introduction of nanofillers into polyamide and polyolefin blends led to a much higher increase in the barrier properties than if the nanofillers were introduced into the polyamide alone. In other words, if polyamide and polyolefin blends are compared with the same polyamide and polyolefin blends but containing nanofillers, the increase in the barrier properties is much greater than if a polyamide is compared with the same polyamide but containing nanofillers. Furthermore, the polyamide and polyolefin blends containing nanofillers are more efficient in real value as a barrier than the same polyamides containing nanofillers but containing no polyolefin (the proportion of nanofillers relative to the polyamide being the same). These very good barrier properties concern numerous technologies such as for example SMC, fumigation, expandable polystyrene packaging and air conditioning fluids. The property of a barrier to oxygen is useful in food packaging.